1. Field of the Invention
The invention is related generally to the use of resistivity measurements for identification of fracturing and determination of the extent of fracturing in earth formations.
2. Background of the Art
In exploration for hydrocarbons, a significant number of reservoirs involve fractured reservoirs. Broadly speaking, there are two types of situations encountered in development of such reservoirs. The first case involves a rock matrix that has a significant porosity so that the hydrocarbons occur within the pore spaces of the rock matrix; however, the permeability of the matrix itself is very low, making development of such reservoirs uneconomical. In such rocks, permeability resulting from fracturing of the rock matrix may make commercial development economical. A second case involves reservoirs in which the only significant porosity in the reservoir is due to fracturing of the rock matrix. Examples of reservoirs that produce from fractured granite are the Playa Del Rey field and the Wilmington field in California, and the Hugoton field in Kansas. It is thus important to be able to identify the extent of fracturing in earth formations.
Fractures observed in boreholes hold important clues for the development of a field. Open natural fractures may enhance productivity in the case of depletion drive or lead to early water breakthrough under a water drive or strong aquifer scenario. However, cemented fractures may form barriers to flow. Therefore it is important to know the orientation and density of the natural fractures to allow for optimized field development. Drilling induced fractures can also be observed in a wellbore. This information can be used to determine the direction in which hydraulic fractures employed in the development of tight reservoirs will propagate. The actual hydraulic fractures can be monitored with micro-seismic, which is relatively expensive and requires a monitoring well close by.
U.S. Pat. No. 4,831,600 to Hornby et al. teaches a method for determining the relative depth and width of fractures which intersect a borehole. Specifically, a sonic logging sonde having an acoustic source and at least one acoustic detector is deployed in the borehole. The source generates a tube wave, commonly referred to as a Stoneley wave, which propagates through the borehole. Based on the travel time and energy content of the Stoneley wave produced by the acoustic source as received by the detector, the depth and/or width of a fracture which has intersected the borehole can be determined. U.S. Pat. No. 5,616,840 to Tang teaches a method for modeling fracture zones in the sidewall of a borehole and for estimating the hydraulic conductivity thereof. The method first separates Stoneley wavefields into a directly-transmitted wavefield and a one-way (i.e. downgoing) reflected wavefield from which the depth configuration may be determined. The separated wavefields are corrected for the effects of borehole irregularities due to such effects as washouts and the like by numerically modeling Stoneley wave propagation using caliper and slowness measurements. The hydraulic conductivity is estimated from parameters derived from synthetic and measured Stoneley wave data across the fracture zone in combination with measurements of the borehole radius. The propagation of hydraulic fractures can be monitored with micro-seismic, which is relatively expensive and requires a monitoring well close by.
The methods of Hornby et al. and of Tang are limited in their ability to provide an estimate of the extent (distance from the borehole into the formation) of the fractures. U.S. Pat. No. 5,243,521 to Luthi teaches the use of a formation microscanner for fracture analysis. The formation microscanner comprises a multi-electrode pad arrangement for providing a resistivity image. The Luthi method requires manual selection of the fractures to be examined in order to exclude the effects of drilling induced micro-fractures on the aperture calculation. The Luthi method cannot provide results in real time in the field.
Shallow resistivity imaging tools have azimuthal sensitivity and can thus identify features such as fractures at the borehole wall. However, they lack the ability to see deeper into the formation, something that is essential to delineate the radial extent of fractures. Prior art deep resistivity measurements, on the other hand, generally do not have azimuthal sensitivity. For galvanic measurements, the 360° design of the current and measurement electrodes provided an axial symmetric tool response. The same is true for conventional induction tools with vertical transmitter and receiver coils.
U.S. Pat. No. 6,466,872 to Kriegshauser et al. having the same assignee as the present application and the contents of which are fully incorporated herein by reference discloses use of a multi-component logging tool for determination of anisotropic resistivity parameters of a laminated reservoir. As would be known to those versed in the art, such a laminated reservoir that has layers of different resistivities exhibits transverse isotropy even if the layers themselves are isotropic. Such a multicomponent logging tool has azimuthal sensitivity. The present invention is based on the recognition that a multicomponent logging tool such as that described in Kriegshauser could have the ability to delineate the extent of fracturing, particularly vertical fracturing, in earth formations.